By ADAM ENTOUS,JULIAN E. BARNES and JAY SOLOMON

WASHINGTON—U.S. defense leaders are increasingly concerned that Israel is preparing to take military action against Iran, over U.S. objections, and have stepped up contingency planning to safeguard U.S. facilities in the region in case of a conflict.

President Barack Obama, Defense Secretary Leon Panetta and other top officials have delivered a string of private messages to Israeli leaders warning about the dire consequences of a strike. The U.S. wants Israel to give more time for the effects of sanctions and other measures intended to force Iran to abandon its perceived efforts to build nuclear weapons.

Stepping up the pressure, Mr. Obama spoke by telephone on Thursday with Israeli Prime Minister Benjamin Netanyahu, and U.S. Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, will meet with Israeli military officials in Tel Aviv next week.

The high-stakes planning and diplomacy comes as U.S. officials warn Tehran, including through what administration officials described Friday as direct messages to Iran's leaders, against provocative actions.

Tehran has warned that it could retaliate to tightened sanctions by blocking oil trade through the Strait of Hormuz. On Thursday, Iran's Supreme Leader Ayatollah Ali Khamenei vowed to punish the perpetrators of the assassination—blamed by Iran on the U.S. and Israel—of an Iranian scientist involved in the nuclear program.

The U.S. denied the charge and condemned the attack. Israel hasn't commented.

The U.S. and Iran, however, have taken steps in recent days apparently designed to ease tensions. Iran has agreed to host a delegation of United Nations nuclear inspectors this month. The U.S., meanwhile, has twice this month rescued Iranian sailors in the region's seas.

Covert efforts by Israel's intelligence service to prevent Tehran from developing nuclear weapons have been credited with slowing the program without the high risk of military conflict that could be sparked by an airstrike. Iran says its nuclear program is for peaceful uses.

But Israel has declined to rule out a strike, as has the U.S.

"It is the policy of the Israeli government, and the Obama administration, that all options remain on the table. And it is crucial that the ayatollahs in Tehran take this policy seriously," said Michael Oren, Israel's ambassador to the U.S.

Mr. Netanyahu said in a recent interview that Iran has begun to "wobble," a signal some U.S. officials believe suggests he is willing to follow the current U.S. strategy, which seeks to avoid a military confrontation with Iran.

"Recent comments by the Israelis show they understand how tough the sanctions we've put in place are and are giving them time to work," said a senior Obama administration official.

The U.S. military is preparing for a number of possible responses to an Israeli strike, including assaults by pro-Iranian Shiite militias in Iraq against the U.S. Embassy in Baghdad, according to U.S. officials.

The U.S. believes its embassy and other diplomatic outposts in Iraq are more vulnerable following the withdrawal of U.S. forces last month. Up to 15,000 U.S. diplomats, federal employees and contractors are expected to remain in Iraq.

In large measure to deter Iran, the U.S. has 15,000 troops in Kuwait, and has moved a second aircraft carrier strike group to the Persian Gulf area.

It has also been pre-positioning aircraft and other military equipment, officials say. Arms transfers to key allies in the Gulf, including the United Arab Emirates and Saudi Arabia, have been fast-tracked as a further deterrent, officials say.

Israeli officials said Mr. Netanyahu's government continues to closely coordinate with the U.S. in responding to the Iranian threat. "Israel believes that heightened sanctions combined with a credible military threat may dissuade the Iranian regime from developing nuclear capabilities," Mr. Oren said.

Mr. Panetta and other top officials have privately sought assurances from Israeli leaders in recent weeks that they won't take military action against Iran. But the Israeli response has been noncommittal, U.S. officials said.

U.S. officials briefed on the military's planning said concern has mounted over the past two years that Israel may strike Iran. But rising tensions with Iran and recent changes at Iranian nuclear sites have ratcheted up the level of U.S. alarm.

"Our concern is heightened," a senior U.S. military official said of the probability of an Israeli strike over U.S. objections.

Tehran crossed at least one of Israel's "red lines" earlier this month when it announced it had begun enriching uranium at the Fordow underground nuclear facility near the holy city of Qom.

The planned closing of Israel's nuclear plant near Dimona this month, which was reported in Israeli media, sounded alarms in Washington, where officials feared it meant Israel was repositioning its own nuclear assets to safeguard them against a potential Iranian counterstrike.

Despite the close relationship between the U.S. and Israel, U.S. officials have consistently puzzled over Israeli intentions. "It's hard to know what's bluster and what's not with the Israelis," said a former U.S. official.

Inside the Israeli security establishment, a sort of good cop, bad cop routine, in which Israeli officials rattle sabers amid a U.S. scramble to restrain them, has assumed its own name: "Hold Me Back."

Some American intelligence officials complain that Israel represents a blind spot in U.S. intelligence, which devotes little resources to Israel. Some officials have long argued that, given the potential for Israel to drag the U.S. into potentially explosive situations, the U.S. should devote more resources to divining Israel's true intentions.

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—Charles Levinson and Siobhan Gorman contributed to this article.

Write to Adam Entous at adam.entous@wsj.com and Jay Solomon at jay.solomon@wsj.com